The present invention relates to an apparatus for carrying semiconductor wafers during the wafer cleaning process. During the semiconductor manufacturing process, the surfaces of wafers become contaminated with cutting and polishing residue, organics, metals and cleaning solution residue. Even extremely minor quantities of contaminants can negatively affect the wafer for subsequent handling steps or when it is used as a substrate for an electronic circuit. Many devices are on the market for effecting such cleaning and have been generally effective. However, as wafer specifications have become more stringent, so have the wafer cleaning requirements sometimes resulting in a lower yield rate. Further, price competition in the wafer and electronic device markets has also become more intense making manufacturing efficiencies even more important. Thus, scrap needs to be reduced as much as possible to help improve yield rate and maintain profitability.
A wafer can be cleaned by either or both of a spray wash and an immersion wash. While such devices and methods have been generally effective, more stringent specifications have resulted in a higher reject rate for the wafers and semiconductor devices being made from the wafers. Cleaning typically includes sequential cleaning steps, as are known in the art. Each cleaning phase is generally followed by a rinse step to remove the cleaning solution. In a sense, the rinse step is also a cleaning step whose chief purpose is to remove the cleaning solutions. Generally, the first cleaning step involves the application of a base such as ammonia followed by a water rinse step. After the first rinse step, the wafers are exposed to an acid such as hydrofluoric, fluoric or hydrochloric. If there is any ammonia carried with the wafers to the acid cleaning step, it will react with the acid and produce a salt which is a contaminant to the wafer. The acid treated wafers are rinsed again with water. After this subsequent water rinse, the wafers are exposed to isopropyl alcohol (IPA) in a vapor chamber to assist in removing the rinse water and to dry the wafers. All of the cleaning fluids must be extremely clean so as to not contaminate the wafers. Such cleaning process may be used at more than one point in the wafer manufacturing process.
During cleaning, wafers are carried in a carrier. The wafers are robotically placed in grooves in carrier rods in the cassette. The grooves retain the wafers in position in the cassette while exposing as much of the wafer surfaces as practicable to the cleaning and rinsing fluids. It has been found that the surfaces of the grooves will induce the formation or collection of residue from the cleaning and/or rinsing liquids on the wafers in the area where the faces of the wafer are adjacent the groove surfaces at a marginal edge of the wafer.
The use of grooves has been found to be particularly advantageous for holding wafers in position in the cassette. To robotically load and unload the wafers, the wafers must be accurately positioned which is accomplished by the use of narrow grooves in cassette rods. Thus, to continue to use current robotics and wafer processing equipment, narrow grooves have been preferred for accurately positioning the wafers. The grooves, although effective for use with robots, are believed to be a major contributor to the formation of the aforementioned spotting problem. Spots form on marginal edge portions of wafers where they are closely spaced from or in contact with carrier rod groove surfaces. Even though at one time the spots would have been considered minor defects, they need to be eliminated or reduced to improve the yield rate to maintain acceptable pricing and margins. It would also be desirable to continue to use cassettes with grooved rods because of their effectiveness at wafer retention, support and positioning which are also important to effective and efficient manufacturing.
The spots are a film or residue left when the cleaning and rinsing liquids are removed from wafers. It has been found that the incident rate of spot creation can be reduced by providing better drainage of cleaning and rinse liquids and more IPA in the area between the groove surfaces and the marginal edge portions of the wafer positioned in the carrier. This can be done while still providing the desired degree of wafer position tolerance in the cassette during cleaning.
Current cassettes have frames that are typically made of fused quartz which is expensive and make the cassettes difficult to repair. Fused quartz is desirable because it is resistant to degradation by the cleaning and rinsing fluids. Many times the cassettes will use grooved carrier rods made of a polymer that is resistant to degradation by the cleaning fluids. However, polymers acceptable from a degradation standpoint have tended to be soft and not resistant to wear caused by contact with the wafers. In order to obtain acceptable life from a wear standpoint, the area that contacts the wafers was made large, making the spotting worse. Carrier rods are also lacking in structural rigidity because of the properties of the polymer and need to be reinforced to support loads. A typical carrier rod is hollow polytetrafluoroethylene with, e.g., a graphite composite rod sealed inside for structural support. However, after some use, such carrier rods tend to leak cleaning fluid which attacks the reinforcing rod and contaminate the wafers being cleaned.
Thus, there is a need for an improved cassette and carrier rod. The present invention provides an improved cassette and carrier rod that will improve cleaning efficiency and thereby improve the wafer production yield rate while maintaining accurate positioning of the wafers so they can be handled robotically.